1. double faulting
1. hitting an unforced error on second serve returns
(they are tied for #1)
3. not getting a high percentage of first serves in
4. not following the deep-to-deep, short-to-short principle (e.g., overusing the low percentage shot down the alley around the up man)
5. lack of communication between partners (e.g. no verbal help at all on switching, calling 'out' balls, deciding on a strategy for middle balls, etc)
6. guarding/hugging the line when your partner is serving
7. expecting your partner to cover all lobs
8. failure to put away either volley or overhead sitters (the worst case of this is actually committing an UE on a sitter)

I coach high school teams in both the spring and fall. Having done this for a number of years now, there are definitely a few sins that we have to coach the kids away from on a regular basis. In no particular order:

When the opponents are in the one up-one back formation, hit away from the player at the net. We probably repeat this more than anything else.

Some sluggers like to use those big topspin strokes from the baseline (with lots of net clearance) for doubles because it's what they know from their training. In a doubles setting though, those strokes often leave the ball up around shoulder height and a poaching opponent with some awareness will gobble them up. If they can't keep the ball away from an opponent up at the net, they have to at least keep the ball LOW.

I constantly remind my troops to mind the wind. Chuck a defensive lob up in the air with the wind on your back and that shot will usually fly long. No downwind lobs.

When it's feasible, avoid getting stuck in the one up-one back formation.

You can hit big topspin loopy groundies up the middle, but you better keep it lower than a foot as it crosses the net.
Any lob that the overheader hits within his service line.
And serving out wide each and every single time.
Move too early to poach, or failing to recognize what your opponent is seeing.

One of my biggest pet peeves is a doubles partner in the rear court who does not switch sides when the net player had moved across the center service line to poach. Countless times I have moved across to poach the ball only to discover that my partner has not budged from his position -- he is standing in the backcourt behind me. If I hit an outright winner, we are ok. Quite often, however, I will hit a poach shot that elicits a very weak x-court response from the opponent. If my partner had moved to the correct position, he could possibly have hit a winner on the next shot. Instead, the opponent will often get a free point with their weak reply because there is no one there to make a play the ball. Occasionally, my poach will not be as damaging as I had planned and the opponent is able to hit an easy winner because my partner has left quite a bit of the court open by failing to switch.

Another pet peeve is related to the first. In this situation, the net player moves across the center service line to poach the ball. Seeing this, I move to cover the other side -- I will sometime move across the baseline but usually I am moving forward to join my partner at the net. As I am moving to cover the other side my partner decides to move back across the center line toward his original position. Since I am further from the net than my partner (even if I am moving forward to take the net), it is not possible for me to change direction in time to effectively cover the side that my partner has just abandoned.

And then there is the net player who moves to poach the ball and then camps out on the center line near the net. As their partner I have no clue whether I should move left or right and we end up in an "I" formation. Now if the net guy has hit a very effective shot with his poach, he might be justified, in some cases, hanging out in the middle because he is absolutely certain that he can put the ball away with his next shot. Once in a while this backfires tho'.
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Yeah sure, i'll vent! But first, to preface, that in order to get indoor time to hit with, i play with what i call, the 3.75's.. they THINK they are 4.0 players, but are far from it!.. I have one buddy and me who are ringers there, but we are frequently separated, and then THIS occurs! haha
a)When I hit a HUGE serve down the T on either side that the returner barely gets his racket on, and my sad 3.5 doubles player i was stuck with just SITS THERE AND WATCHES!(what should be done, is he should POACH, EVERY D*MN TIME!!!).. Its his point to WIN, every time, and he just then forces me to start the point, instead of him to END IT!!.. ok.. that is numero UNO to me!(I quite often feel like i am literally playing ONE AGAINST TWO!.. because, I AM!!!)
b)My partner CAN'T VOLLEY, or volleys very badly, and won't take LESSONS to learn how to or get better!.. So, again, lends to them standing on the inside singles line, just hoping the ball goes nowhere NEAR THEM until they can be safe back to the baseline!
c)Then, when they serve, their serves, first and second, are TOO WEAK for me to poach, and thus help out my sad partner!.. If i try to 'gamble'(as it is at this point), then the returner has PLENTY of time to make ME look SILLY and pass me down the line or just bunt it cross court.. and oh, while we are at it...
d)Those same weak az servers serve too close from the middle of the court, and don't even have the sense to SLIDE OUT for the return that surely will be angled out wide to them to which they won't even get TO the ball, or we will then be on the DEFENSE!.. pathetic.. happens all the time, even with me begging for them to stand farther out/slide out after serve.
e)players that then DON'T KNOW WHERE TO STAND, and then look at me like I AM CRAZY when i try to suggest where to stand.. that is when i know it will b a LOOONG day playing with the fool.
f)players that don't understand angles, and guarding the best possible shot the other player has, again, just standing in one "safe spot" just CLUELESS about what to do and where to stand to even START being an effective doubles player!(my buddy is easilly 20 to 30 years older than these guys, but can volley, understands all these concepts, and we regularly BAGEL these horrible tennis players!)
The only thing apparently that keeps these guys together are their egos and the fact that they all suck at the same level together. I enjoy having open targets from their cluelessness, but then when I get one on my team, i want to run a CLINIC for all of them, so maybe they will not be so horribly bad and make me feel like i am playing one on two, which even seems worse than that on some match-ups. I will try to hold back my suggestions, but some days just cant, and the guy will go, "Just play your game, and I will play mine...", and i think.."I wish I could CLONE MYSELF and keep YOU TOTALLY OUT OF THE EQUATION!" haha
That covers most of it, with a lack of a)serve, b)volley, these guys simply play doubles because indoor time is too expensive to play singles is how I see it.(and I would be equally scared to watch them play singles)
oh, and my last one i almost forgot about.. i serve big, the ball FLOATS TO THEM, they don't bother to even take ONE STEP BACK to attempt an overhead, and then leave me, thinking why wouldn't they take this easy shot, rushing to position behind them to hit the next shot!.. oh, I realize I could go on and on.. but these are just the tip of the bad doubles ICEBERG!.. haha

Not supporting and encouraging a partner who's missing.
You don't have to give pep talks, but don't point out his mistakes with a lesson at the end.
Praise his good shots.

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Totally agree...No matter how pissed I am with my partner's game, I would not point out his mistakes at the end of the match. Even when he asked of my opinion, I would try to be diplomatic with my answer on suggestions of ways of hitting. Another no-no is to correct your partner every mistake he makes during the game.

One of my biggest pet peeves is a doubles partner in the rear court who does not switch sides when the net player had moved across the center service line to poach. Countless times I have moved across to poach the ball only to discover that my partner has not budged from his position -- he is standing in the backcourt behind me. If I hit an outright winner, we are ok. Quite often, however, I will hit a poach shot that elicits a very weak x-court response from the opponent. If my partner had moved to the correct position, he could possibly have hit a winner on the next shot. Instead, the opponent will often get a free point with their weak replay because there is no one there to make a play the ball. Occasionally, my poach will not be as damaging as I had planned and the opponent is able to hit an easy winner because my partner has left quite a bit of the court open by failing to switch.

Another pet peeve is related to the first. In this situation, the net player moves across the center service line to poach the ball. Seeing this, I move to cover the other side -- I will sometime move across the baseline but usually I am moving forward to join my partner at the net. As I am moving to cover the other side my partner decides to move back across the center line toward his original position. Since I am further from the net than my partner (even if I am moving forward to take the net), it is not possible for me to change direction in time to effectively cover the side that my partner has just abandoned.

And then there is the net player who moves to poach the ball and then camps out on the center line near the net. As their partner I have no clue whether I should move left or right and we end up in an "I" formation. Now if the net guy has hit a very effective shot with his poach, he might be justified, in some cases, hanging out in the middle because he is absolutely certain that he can put the ball away with his next shot. Once in a while this backfires tho'.

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Oh man....I had this partner at last night's game. I have to console myself my partner is not a doubles' player and try to avoid him whenever I could.

Not moving, and not taking an offensive position near the net is one of the big problems in low level doubles. What the Bryan brothers said in one of their videos is now stuck in my head like a broken record... "there are no hard volleys", with the unsaid part being "if you take the right offensive position". The other thing that I repeat to myself is, "always expect the ball the come to me", which forces me to split step on every shot and be ready for the next one.

Don't get mad at your partner, encourage them. I've seen when I Gert mad at my partner or vice versa both of us start playing worse, you gotta keep your cool go back to the basics.

Don't poach every time or you will start getting burned down the line

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This is rarely a problem I see. What I see a lot more of are net players who act as if their feet are made of concrete or as if their feet were actually embedded in the court. Not only do they not poach, they don't even fake poach so the returner doesn't even have to consider that a poach is an option they'll have to deal with.

IMO the server's partner should be poaching or fake poaching on every first serve and on a high percentage of second serves. You've got to get into the receiver's head and if you mix it up not only will the quality of their returns go down but they'll even cough up the occasional unforced return error which is just gravy.